1. Field of the Invention
The present invention relates to a device and method for photoelectrically converting light into an electrical signal wherein a potential at a control electrode region of a semiconductor transistor is controlled through a capacitor to store carriers excited by means of light in the control electrode region, thereby controlling an output from the semiconductor transistor.
2. Related Background Art
FIG. 1A is a plan view of a photoelectric transducer device described in European Patent Provisional Publication No. 132076, FIG. 1B is a sectional view of the device taken along the line I--I thereof, and FIG. 1C is a diagram of an equivalent circuit thereof.
Referring to FIGS. 1A to 1C, photoelectric transducer cells are arranged on an n-type silicon substrate 101. The photoelectric transducer cells are electrically insulated by an element isolation region 102 made of SiO.sub.2, Si.sub.3 N.sub.4 or polysilicon.
Each photoelectric transducer cell has the following construction. A p-type impurity is doped to form a p-type region 104 on an n.sup.- -type region 103 having a low impurity concentration formed by an epitaxial technique or the like. An n.sup.+ -type region 105 is formed in the p-type region 104 by impurity diffusion or ion implantation. The p- and n.sup.+ -type regions 104 and 105 serve as the base and emitter of each bipolar transistor.
An oxide film 106 is formed on the n.sup.- -type region 103. A capacitor electrode 107 having a predetermined area is formed on the oxide film 106. The capacitor electrode 107 opposes the p-type region 104, interposing the oxide film 106 therebetween, thereby constituting a capacitor C.sub.ox. Upon application of a pulse voltage to the capacitor electrode 107, the potential of the p-type region 104 in the floating state is controlled.
An emitter electrode 108 connected to the n.sup.+ -type region 105, a wire 109 for extracting the signal from the emitter electrode 108, and a wire 110 connected to the capacitor electrode 107 are formed at the upper surface side of the substrate 101. An n.sup.+ -type region 111 having a high impurity concentration and an electrode 112 for applying a voltage to the collector of a bipolar transistor are sequentially formed on the lower surface of the substrate 101.
The basic operation of the photoelectric transducer device will be described below. Assume that the p-type region 104 as the base region of the bipolar transistor is set in the initial state of a negative potential. Light 113 is incident on the p-type region 104 and carriers corresponding to the amount of light are stored in the p-type region 104 (storage operation). The base potential is changed by the charged carriers. The change in base potential controls a current supplied to an emitter-collector path, and an electrical signal having a level corresponding to the amount of incident light is extracted from the floating emitter electrode 108 (read operation). In order to remove the carriers stored in the p-type region 104, the emitter electrode 108 is grounded and a refresh positive voltage pulse is supplied to the capacitor electrode 107. Upon application of this positive voltage, the p-type region 104 is forward-biased with respect to the n.sup.+ -type region 105, thereby removing the charged carriers. When the refresh pulse falls, the p-type region 104 returns to the initial state of the negative potential (refresh operation). The cycle of storage, read, and refresh operations is repeated.
According to the method proposed here, the carriers generated upon reception of light are stored in the p-type region 104 as the base region, and the current supplied between the emitter and collector electrodes 108 and 112 is controlled by the stored carriers. Therefore, the stored carriers are amplified by the amplification function of each cell, and the amplified carriers are read out. Therefore, a high output with high sensitivity and low noise can be obtained.
A potential Vp generated by the base by the carriers stored in the base upon light excitation is given by Q/C where Q is the charge of the carriers stored in the base, and C is the capacitance connected to the base. As is apparent from the above mathematical expression, both Q and C are reduced according to the reduction of the cell size when the transducer device is highly integrated. It is thus found that the potential Vp generated by light excitation is kept substantially constant. Therefore, the method proposed by the above prior art is advantageous in a future high-resolution implementation.
A change in base potential Vb during the application of the positive refresh voltage to the capacitor electrode 107 is given as follows: EQU (Cox+Cbe+Cbc)dVb/dt=-Ib
where Cbe is the capacitance between the base and emitter of the bipolar transistor, Cbc is the capacitance between the base and collector thereof, and Ib is the base current.
FIG. 2 is a graph showing changes in base potential Vb during the application of the positive refresh voltage as a function of time.
Referring to FIG. 2, the initial base potential at the time of the application of the refresh pulse varies according to the magnitude of the storage voltage Vp. The negative potential in the initial state is changed in the positive direction by the storage voltage Vp upon the storage operation. In this state, when the positive refresh pulse is applied to the capacitor electrode 107, the initial base potential is increased by the storage voltage Vp.
As is also apparent from the graph in FIG. 2, the time for maintaining the initial base potential varies according to the magnitude of the initial base potential. However, after the lapse of this period, the base potential Vb is decreased at a constant rate regardless of the initial base potential. If the refresh time t is sufficiently long, the base potential Vb can be controlled to be substantially 0 V regardless of the magnitude of the storage voltage Vp. Therefore, the base potential Vb returns to the predetermined negative potential of the initial state when the refresh pulse falls.
However, in order to achieve high-speed operation, the refresh operation is terminated at the refresh time t=t0 and the base potential Vb=Vk in practice. Even if a residual potential of the base potential Vb is present, the base potential Vb can return to the predetermined negative potential when the refresh pulse falls under the conditions wherein the refresh time t=t0 is established and the base potential Vb is constantly the predetermined potential Vk. Therefore, the negative potential can be set to be the initial state.
However, in the conventional photoelectric transducer device described above, when the refresh operation is repeated, the residual potential Vk is gradually decreased and undesirably causes an after-image phenomenon.
Referring to FIG. 2, assume that the initial base potential of a cell receiving a large amount of light is 0.8 V and that the initial base potential of a cell receiving a small amount of light is 0.4 V. When the refresh time the has elapsed, the base potential Vb of the cell receiving a large amount of light becomes the predetermined residual potential Vk. However, the base potential Vb of the cell receiving a small amount of light becomes a residual potential Vl lower than the predetermined residual potential Vk. In this state, if the refresh pulse rises, the base potential Vb of the cell receiving a small amount of light becomes lower than the negative potential of the initial state. From a potential lower than the initial negative potential, storage and read operations are started. Therefore, if the refresh operation is repeated in a low illuminance state, the residual potential of the base is gradually decreased. Even if a high illuminance state is obtained, the resultant output has a level lower than that corresponding to the amount of incident light. In other words, the after image phenomenon occurs due to the following reason.
When the refresh operation is repeated, the holes in the base region are recombined to result in their shortage. If the shortage of holes cannot be compensated for a long period of time, i.e., if the low illuminance state continues for a long period of time, the after image phenomenon typically occurs.
In a conventional photoelectric transducer device, if its cells are arranged in a matrix form, the following blooming phenomenon also occurs undesirably.
FIG. 3 is a schematic circuit diagram of an area sensor using a conventional photoelectric transducer device. Referring to FIG. 3, conventional photoelectric transducer cells 120 of a 3.times.3 matrix are arranged in the area sensor. Emitter electrodes 108 of the cells 120 are connected to corresponding vertical lines in units of columns. Capacitor electrodes 107 are connected to horizontal lines in units of rows. A positive voltage from a vertical scanning unit 121 is applied to the photoelectric transducer cells 120 to perform read access or refresh operation in units of rows.
If a given photoelectric transducer cell receives a large amount of light and the potential of the base region 104 is higher than that of the corresponding emitter region 105, the potential of the vertical line connected to this emitter electrode 108 is increased although read access is not performed. If other photoelectric transducer cells, the emitter electrodes 105 of which are connected to the same vertical line, are subjected to read access, a readout signal is output to the corresponding vertical line although the given cell does not receive light. In other words, the blooming phenomenon occurs along the vertical direction.